pp_match(): remove redundant condition

a successful match always sets $-[0] now, so there's no need to check
whether its set
pp_match(): only look up pos() magic once

Currently before matching, we see whether the SV has any pos() magic
attached; then after the match we look it up again to update pos().
Instead just remember the previous value of mg and reuse it where
possible.
pp_match(): remove some superfluous braces